Don't ICE on bound types in sized conditions

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -2148,12 +2148,13 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 }))
             }
 
-            ty::Alias(..) | ty::Param(_) => None,
+            ty::Alias(..) | ty::Param(_) | ty::Placeholder(..) => None,
             ty::Infer(ty::TyVar(_)) => Ambiguous,
 
-            ty::Placeholder(..)
-            | ty::Bound(..)
-            | ty::Infer(ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => {
+            // We can make this an ICE if/once we actually instantiate the trait obligation.
+            ty::Bound(..) => None,
+
+            ty::Infer(ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => {
                 bug!("asked to assemble builtin bounds of unexpected type: {:?}", self_ty);
             }
         }

tests/ui/traits/non_lifetime_binders/bad-sized-cond.rs

@@ -0,0 +1,13 @@
+#![feature(non_lifetime_binders)]
+//~^ WARN is incomplete and may not be safe
+
+pub fn foo()
+where
+    for<V> V: Sized,
+{
+}
+
+fn main() {
+    foo();
+    //~^ ERROR the size for values of type `V` cannot be known at compilation time
+}

tests/ui/traits/non_lifetime_binders/bad-sized-cond.stderr

@@ -0,0 +1,28 @@
+warning: the feature `non_lifetime_binders` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/bad-sized-cond.rs:1:12
+   |
+LL | #![feature(non_lifetime_binders)]
+   |            ^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #108185 <https://github.com/rust-lang/rust/issues/108185> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+error[E0277]: the size for values of type `V` cannot be known at compilation time
+  --> $DIR/bad-sized-cond.rs:11:5
+   |
+LL |     foo();
+   |     ^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `V`
+note: required by a bound in `foo`
+  --> $DIR/bad-sized-cond.rs:6:15
+   |
+LL | pub fn foo()
+   |        --- required by a bound in this
+LL | where
+LL |     for<V> V: Sized,
+   |               ^^^^^ required by this bound in `foo`
+
+error: aborting due to previous error; 1 warning emitted
+
+For more information about this error, try `rustc --explain E0277`.